In the step of diffusing a dopant in production processes of semiconductors, a pattern forming step of a protective film using a photolithography technique is first carried out, and subsequently the selective diffusion step of an intended dopant into the substrate (doping) is carried out by thermal diffusion or ion implantation using the protective film as a mask.
More specifically, for example, a resist is coated on the surface of a silicon substrate which was entirely covered with an oxide film. Next, thus formed resist film is exposed and developed using a photolithography technique, thereby removing the resist film selectively. Thereafter, the oxide film at the part where the resist film was removed is removed by etching to allow the silicon substrate to be exposed. A diffusive dopant is supplied by vapor deposition or the like onto the silicon substrate to accumulate the dopant. The remaining resist is removed to produce a semiconductor element.
In this production process, diffusion of the dopant, such as phosphorus having five valence electrons, into the silicon substrate on which the resist film and the oxide film were removed leads to generation of a free electron from one residual electron. Accordingly, the part of the silicon substrate in which the dopant is diffused serves as a semiconductor. Therefore, it is necessary to protect with the protective film at the part where the diffusion of the dopant should be avoided so as not to permit diffusion of the dopant. In the foregoing process, a silicon oxide film plays a role as the protective film.
Meanwhile, solar batteries include a junction of a p-type semiconductor and an n-type semiconductor (p-n junction). When a light is irradiated on the semiconductor, a combination of an electron (−) and a positive hole (+) is generated. The generated electron and positive hole are accumulated on both sides of the junction. The light entered in the solar battery produces an electric current and a voltage, and the electric current and the voltage produced by the light can be used as an electric power.
When, for example, a p-type semiconductor is provided on one surface of such a solar battery, a dopant having three valence electrons is diffused. In this process, it is necessary to protect another surface by a protective film so as not to permit diffusion of the dopant.
As the protective film which can be used in such a process, for example, a protective film including a diffusion inhibitor that includes at least one selected from titanium, titanium oxide and a glass is disclosed (see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-158277